The wind, of itself and unassisted by hard particles, can accomplish but little in disintegrating firm rocks, but on high mountain crests and "knife-edges," where the wind blows with great velocity, it may accomplish considerable destruction. When, however, the wind is able to drift along quantities of sand and fine gravel, it becomes a disintegrating agent of importance. Except on sandy coasts, this agency is of small efficiency in regions of ordinary rainfall, because in these the soil is protected and held together by its covering of vegetation. On sandy coasts we may often observe the abrading effects of wind-driven sand. In a Cape Cod light-house a single heavy gale so ground a plate-glass window as to render it opaque and useless, and on that same coast window-panes are sometimes drilled through by the sand flying before a storm. Fragments of glass lying on the sand dunes are soon worn as thin as a sheet of paper.
Fig. 44. - Exfoliating granite dome, Yosemite Valley, California. ^U. S. G. S).
In arid regions, and more especially in sandy deserts, high winds sweep along much sand and fine gravel, which are hurled against any obstacle and gradually cut it away.
Very hard rocks yield but slowly to the cutting action of wind-driven sand, and in them the chief effect to be observed is a scratching and polishing of the surface. The same principle is employed in the sand-blast, which is a jet of sand, driven at a ' high velocity and used to engrave glass, polish granite, and do other work of the kind. Soft rocks are quite rapidly abraded and cut down by the drifting sand, and go to increase the mass of cutting material. The softer parts are cut away first, leaving the harder layers, streaks, or patches standing in relief. In this way very fantastic forms of rocks are frequently shaped out; pot-holes and caverns are excavated by the eddying drift, and archways cut through projecting masses. (See Frontispiece and Figs. 45-46).
Fig. 45. - Wind-sculptured sandstone, Black Hills, South Dakota. (U. S. G. S).
As the wind does not lift the harder and heavier particles to any great height, the principal effect is produced near the level of the ground, and thus masses of rock are gradually undermined and fall in ruins, which in their turn are slowly abraded. Isolated blocks are sometimes so symmetrically cut away on the under side, that they come to rest upon a very small area and form rocking stones, which, in spite of their size and weight, may be swung by the hand.
Fig. 46. - Honey-combed rock, due partly to wind erosion and partly to the solvent action of rain. (U. S. G. S).
The fine particles abraded from the rocks by drifting sand have undergone no chemical change, the process being entirely mechanical.
The abrading effects of wind-driven sand may be observed in any desert region where naked rocks are exposed, as, for example, in the arid parts of Utah and Arizona. One very characteristic effect of this natural sand-blast is found in the appearance of the pebbles shaped by it. Pebbles of very hard and homogeneous materials, such as quartz or chalcedony, are highly polished. Those made from igneous rocks have the softer minerals worn away, leaving the harder to stand in relief in curious patterns, while limestone is carved into beautiful arabesques.
The wind-driven sand, which does the work of abrading, is itself abraded and grows finer, the longer the distance which it traverses.
We have seen that the rain is slowly shifting the soil seaward, and in dry countries the wind acts in similar fashion. Strong winds, blowing steadily in one direction, carry great quantities of dust and fine sand with them, sometimes directly into the sea or other bodies of water, sometimes into rivers, or again to moister regions, where it comes under the influence of the rain.
Slowly as they work, the wind and temperature changes prevent any complete stagnation in the circulation of material, and thanks to them, the processes of disintegration of rock and transportation of soil are kept up even in the dryest deserts.